Hydrocarbon processing plants, from refineries to petrochemical plants suffer from fouling as a result of deposition of hydrocarbon foulants onto metallic surfaces of distillation columns, vessels, lines, overheads and other hydrocarbon processing equipment. The hydrocarbon foulants include a wide variety of hydrocarbons that may be present in crude oil as well as the byproducts of hydrocarbon refining processes.
For example, asphaltenes are the heaviest and the most polar components of crude oils. They are generally defined as a solubility class of the polydisperse, high molecular weight hydrocarbons that are insoluble in non-polar solvents. Asphaltene particles are believed to exist in the form of a colloidal dispersion stabilized by other components of the crude oil. These naturally occurring dispersions can be destabilized by a variety of mechanical and chemical conditions involved in oil production and processing. This can result in asphaltene aggregation, precipitation, and eventual deposition of a tarry residue. Other high-molecular weight hydrocarbon foulants include heavy oil, tars, polynuclear aromatic hydrocarbons, coke, and the like.
Other hydrocarbon foulants include polymers, such as those formed from polymerization of styrene, butadiene, cyclopentadiene, and the like, aliphatic and aromatic hydrocarbons having a density less than that of water, commonly referred to as light oil, oxidized hydrocarbons, and thermal decomposition products resulting from the degredation of larger molecules, such as methyl tert-butyl ether, polymers, or other large molecules into smaller molecules.
In ethylene plants, dilution steam systems (DSS) separate and recover ethylene quench water from hydrocarbons, recover heat, and generate steam for pyrolysis furnaces. Dilution steam is essential to reducing the hydrocarbon partial pressure, promoting the formation of ethylene, reducing the formation of undesirable heavier compounds, and reducing coke formation in the furnace tubes. Dilution steam is approximately 50% of the furnace feed. For ethylene units that do not produce enough dilution steam to satisfy the steam-hydrocarbon ratio, about 50 to 150 psig plant steam is then injected into the furnaces.
The DSS incorporates a number of individual functions including process water recovery, hydrocarbon stripping, and dilution steam generation. Each function is closely linked to changes in plant operation i.e., cracking severity, feedstock, and imported or recycle streams.
Ethylene quench water is produced in the quench water tower (QWT) where incoming hot, cracked gas is cooled to a suitable temperature for compression. The cooling is done by spraying cool water from the top of the tower onto upward flowing hot gases. The gases continue to the compression train for processing. These gases contain many molecules that can react and create fouling. This fouling in the compressors can reduce the compressor efficiency. Once enough efficiency is lost, the plant may need to remove the compressor from service to clean it. This can result in an unscheduled shut-down of the ethylene plant.
The gas is often hydrotreated to reduce triple bonds to double bonds. This is typically done with equipment such as an acetylene converter. The converter specifically adds hydrogen molecules to triple bonds creating double-bond molecules. The triple-bond molecules may be highly reactive and easily form heavy, non-volatile molecules that foul the associated equipment.
Major condensation of steam occurs during the quenching operation, which drastically reduces the amount of vapor in the system. In this process a large amount of latent heat is transferred to the process water. This heated process water is used throughout the plant as heating medium, thus recovering a major part of the energy used in the cracking process. A constant low temperature is desired in the top of the QWT.
The high-molecular weight heavy tars that accumulate in the QWT greatly reduce heat transfer, and this affects how well the QWT works. Without efficient heat transfer, the overhead gases enter the compression train at a higher temperature. Once the temperature limit is reached, the rates must be reduced until, ultimately, the plant will need to be shut down to clean the QWT.
After the quenching process, the water stream flows to the QWSD. This water stream is typically a combination of pyrolysis gasoline, process water, recycle quench water, and tars of heavy hydrocarbons. The pyrolysis gasoline in the settler migrates to the top of the drum where it is removed. This stream is commonly known as pygas. The tars or heavy hydrocarbon are usually collected at the bottom of the drum. These are the hydrocarbons that are heavier than water. Not every QWSD is equipped for this phase separation, and in many plants the drain or bottom line(s) may plug because of low flow rates and the heavy, polymer-like composition of the stream.
The process water and recycle quench water need adequate retention time in the QWSD to achieve separation from the hydrocarbon phases. Close to the bottom of the QWSD, the water is pumped away to feed the coalescer unit or the process water stripper (PWS) or both to be further cleaned before steam generation. Hydrocarbon that is carried downstream will reduce the operating efficiency of the downstream units.
Heavy tars accumulate in the bottom of the QWSD, and from a combination of low flow rates, high viscosity, and relatively high freeze points the bottom lines can plug. Once the lines are plugged, the tar builds up, and eventually accumulates enough inventory to affect downstream units.
The heavy tars that accumulate in the QWT and QWSD are notoriously difficult to remove. Consequently, there is an ongoing need for new methods and compositions to effectively remove these foulants in order to prevent system interruptions for cleaning, protect downstream equipment, and increase the overall efficiency of hydrocarbon refining processes.